Re: NAND flash misery



Stefan Reuther wrote:

Vladimir Vassilevsky wrote:

"David Brown" <david@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message

The actual erase block size in NAND flash is something like 32/64/128/256KB,
being bigger for the higher capacity devices. What it implies: any write
operation through the IDE interface is actually read - copy - erase -
modify - write at the controller level.

Not if the implementor of the controller firmware did his homework.

This is what described in the appnotes from Samsung and Sandisk.

If
that's the case, a 512-byte block write ends up as a 512-byte block
write at the flash device, at a new address, and the old address is just
marked as unusable. No read/erase/modify/write for every operation.
Garbage compaction happens a some time in the background.

So something like FAT or MFT has to be maintained internally, and it has to be done in the background instead of by every transaction. This scheme doesn't seem to be very applicable for the removable media.

Since this kitchen is hidden behind IDE, there is no point in
using YAFS or JFS or such.

Exactly. Although I heard some controller firmware used in consumer
flash disks optimizes based on the assumption that the file system at
the other end of the IDE or USB interface is FAT.


A disk cache with the blocks of 32k makes a lot of sense though.
Not more or less for a flash disk than any other disk.

There is a very significant penalty in the speed of the flash write operation at the IDE level if the short blocks are used. The difference can be as high as 10 times or so. According to the appnotes, this happens due to the read - modify - write thing.



Stefan

VLV

.

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